(I dont think this was HIJacking the thread, as it was good info for her to know too.. I'll post here what I posted there)
Phi, your best bet is to get an EQ or a better head unit- what head unit are you running now? Without knowing the speaker type, I can't check the frequency response curve (more RMS at different frequency, for example, my Alpines provide a little more umph at higher midrange than at low range where they start to taper off).
The crossover might be misset- if you're sending frequencies to the tweeters that they can't reproduce that's definetly it. Do you remember the default options? Also check your 4 channel amp. For some reason it might be thinking your speakers are subs and only sending lower frequencies- while your front speakers are classified as "normal" and getting high end.
My Alpine9803 head unit, when "sub" is turned on, sends lower frequency signals with bass amplification to the second set of preouts, when "sub" is off, it's normal preout
the best way to get rocking base is to get a three way system, woofer, midbase and tweeter- but thats not really an option is it?
The JLs should be decent midbase, so lets tackle it from an amplifaction signal direction.
Adding a subwoofer isn't going to cover that high up into the midrange (with the lose you're describe) my JL is good for up to 400hz- but what sub amp is going to send a signal to a sub that high? Not many. So rule out the subwoofers.. not that you shouldn't add one in, but it's not going to cover midbass effectively.
So.. EQ. or check your headunits settings. You're powering the speakers with a 4 channel amp- correct? Good. Eclipse's units advertise 50W peak, but what they go to great lengths to hide from the public is that they only give out 13W RMS. WHich is, pathetic. lol
But you're amping is- so no matter. What you want to do is check the bass center frequency, and the treble center frequency; how to do that on your eclipse I dont know, but it should be able to adjust those settings.. I'll keep thinking and post more later on tonight